A Fischer-Tropsch (FT) process comprises the hydrogenation of CO in the presence of a catalyst based on metals, such as Fe, Co and Ru. The products formed from this reaction are water, gaseous, liquid and waxy hydrocarbons which may be saturated or unsaturated. Oxygenates of the hydrocarbons such as alcohols, acids, ketones and aldehydes are also formed.
A heterogeneous Fisher-Tropsch process may be conveniently categorised as either a high temperature Fischer-Tropsch (HTFT) process or a low temperature Fischer-Tropsch (LTFT) process. The HTFT process can be described as a two phase Fischer-Tropsch process. It is usually carried out at a temperature from 250° C. to 400° C. and the catalyst employed is usually an iron-based catalyst. Generally, the process is commercially carried out in a fluidised bed reactor. It is well-known that HTFT synthesis is preferred for the production of high value linear alkenes, and iron catalysts, operating at high temperatures in fluidised bed reactors, remain the catalysts of choice.
The LTFT process can be described as a three phase Fischer-Tropsch process. It is usually carried out at a temperature from 220° C. to 310° C. and the catalyst employed is usually either a Co-based catalyst or a Fe-based catalyst. The conditions under which this process is carried out, results in the products being in a liquid and possibly also in a gas phase in the reactor. Therefore this process can be described as a three phase process, where the reactants are in the gas phase, at least some of the products are in the liquid phase and the catalyst is in a solid phase in the reaction zone. Generally this process is commercially carried out in a fixed bed reactor or a slurry bed reactor.
LTFT synthesis using Fe-based catalysts is usually the synthesis procedure of choice for the conversion of coal-derived synthesis gas (H2 and CO) to hydrocarbon products. It is also well known in the art that Fe-based catalysts have hydrogenation and water—gas—shift (WGS) activities, the WGS reaction (formula i below) occurring simultaneously with the production of hydrocarbons during hydrocarbon synthesis (formula ii below).CO+H20→CO2+H2  inCO+(2n+1)H2→CnH(2n+2)+nH2O  ii
For commercial reasons it is desirous to operate an FT hydrocarbon synthesis process in the most economical way, taking into account the effect that the WGS reaction has on the hydrocarbon synthesis reaction and vice versa. It is well known in the art that since the water that is formed during the hydrocarbon synthesis is used as a reagent in the WGS reaction and since the WGS reaction consumes CO and produces hydrogen, the ratio of hydrogen to CO in the feed that is used for the hydrocarbon synthesis (syngas) may be higher or lower than the usage ratio
  (      i    .    e    .                  ⁢                  Δ        ⁢                                  ⁢                  H          2                            Δ        ⁢                                  ⁢        CO              )of hydrogen and CO used in the hydrocarbon synthesis reaction.
Accordingly it is known to operate an FT hydrocarbon synthesis process with a syngas having a low hydrogen to CO ratio, typically between 0.5 and 1.3, a high conversion rate and a low water partial pressure in the reactor.
The economically favourable conditions discussed above however impact on the activity and stability of the Iron-based catalyst used in the FT hydrocarbon synthesis process.
Although much research has been undertaken and reported in the art in an attempt to find Iron-based catalysts that are able to perform under the rigours of the above mentioned conditions, the effect of the combination of the manner in which the Iron-based catalyst is activated and the selected hydrocarbon synthesis conditions under which the catalyst is used with a low carboxylic acid selectivity, on the stability and activity of the catalyst and on the hydrocarbon production of the FT hydrocarbon synthesis process have not been the subject of intense study.
Catalysis Today 36 (1997) 325; Canadian J. Chem. Eng., 74 (1996) 399-404; Applied Catalysis. A: General 186 (1999) 255-275; Journal of Catalysis 155, (1995) 366-375 and Energy and Fuels, 10 (1996) 921-926 describe different catalyst activation procedures and their influence on FT synthesis. The influence of different reducing gasses (H2, CO, or a combination of H2 and CO) used during activation is disclosed. Reduction at different pressures and temperatures are also disclosed. However, none of the documents disclose the activation conditions of the present invention.
It will also be noted from the art that little emphasis is placed on the effect of the combination of the manner in which the Iron-based catalyst is activated and the selected hydrocarbon synthesis conditions under which the catalyst is used on the production of oxygenates of hydrocarbon products and more particularly with respect to the production of carboxylic acid, which carboxylic acid has the effect of not only weakening the mechanical integrity of the catalyst but also eroding the equipment used to run the FT synthesis (B-T Teng, C-H Zang, J Yang, D-B Cao. J Chang, H-W Xiang and Y-W Li; Fuel, 84, (2005) 791-800).
The Applicants have now found that the combination of activating the Iron-based catalyst under the conditions set out herein and conducting the FT synthesis under favourable conditions to take into account the WGS reaction allows for an FT synthesis process wherein the hydrocarbon production rate is acceptable with a low production of carboxylic acid and the catalyst activated under the below mentioned conditions has good catalyst life time and activity for a process run under the particular conditions discussed herein.